Was running AL Expeditions for my FLGS. One week, a large group of teenagers showed up looking to join the public play program, so I took them on and ran through DDEX1-1 to show 'em the ropes. Nothing too unusual happened, everyone advanced to level 2, and 4 of them had a good enough time to come back for more the next few weeks.
Cool by me, but one player in particular began to confound me. In hindsight I wonder if he was a brilliant troll, but that's probably just me being paranoid. In reality, I think he just lacked a fundamental understanding about the goals of cooperative play. This was the guy who wandered off on his own, tried to steal anything that wasn't bolted down (and some stuff that was), attacked NPCs without apparent cause or justification, and so on.
One of my favorite things that ever happened with this player was when I ran Embers of Elmwood. The party went to fetch the comatose mystery NPC, and his PC, a dragonborn paladin, simply went up to the guards posted over her and breath-weaponed them. The other PCs (somehow, despite his past behavior) did not see this coming, grabbed the coma NPC and ran like hell. The paladin stayed behind and continued fighting, even when more guards appeared. He eventually wised up and tried to flee, but by that point they had him surrounded and chopped him down.
The player just sorta shrugged it off, said he was going to take off rather than wait around to watch the rest of the session, but that he'd be back next week.
Next week rolls around, and I learn from one of that player's friends that he had recently been banned from the store for shoplifting. Games ran much more smoothly after that.